1. Field of the Invention
The present invention is directed to a tape cartridge and a tape drive, such as a magnetic tape cartridge and a drive of the type known as a tape "streamer," and in particular to a tape cartridge containing multiple tapes and a drive having multiple data transfer heads respectively engageable with multiple tapes.
2. Description of the Prior Art and Related Applications
The need for storing all types of digital information is increasing rapidly. Magnetic tape is by far the least expensive high-capacity medium available today for digital information storage. The suppliers of tape drive and tape medium are always striving to develop new drives with increased storage capacity, shorter access time and higher data rates.
A primary advantage of tape, in addition to its low cost, as an information storage medium is its relatively large recording surface area.
A significant disadvantage of tape as an information storage medium is that tape is a sequential medium. This means that when a data transfer head, such as a read/write head being operated as a read head, is located at a beginning of a tape, it is necessary for the tape to pass along virtually its entire tape length in order to retrieve information at the end of the tape. The access time associated with tape is therefore much longer than the access time associated with hard disks, optical disks and flexible (floppy) disks.
Access time associated with a tape can, at least in theory, be improved by making the tape wider and shorter, so that it takes less time to transport the tape to reach the end of the tape in order to retrieve information located at the end of the tape. There are, however, practical limits to making a tape wider and shorter.
One problem is that the dimensions of the tape, and consequently the dimensions of the cartridge containing the tape, have an influence on the form factor of the drive. A wider tape requires an increased form factor. Consistent with current marketplace desires, the current trend in the industry, however, is to reduce the form factor. Moreover, in library systems the cartridges are stored in magazines, usually with ten cartridges per magazine. Such magazine storage would have to be reconfigured if wider tape, and thus wider cartridges, were employed.
Another problem is that increasing the tape width also increases the air cushion which is unavoidably created between the exterior surface of the tape on the take-up reel and the incoming tape which is being wound onto the take-up reel. In the confined V-shaped space at this location, air which is carried along the surfaces of the rapidly-moving tape becomes entrapped and forms a cushion which pushes the incoming tape slightly away from the wound tape. Increasing the width of the tape would make this trapped volume larger, and thereby increase the cushion effect. This may result in the tape not being firmly wound onto the take-up reel, which can subsequently result in a tape pack shift if the cartridge is subsequently subjected to shock and vibration. As is known to those of skill in this art, a tape pack shift can result in the occurrence of mis-tracking when the tape is subsequently unwound and read.
Another theoretical way to increase the data transfer rate is to use an increased number of write/read heads in parallel. It is well-known, however, that the manufacturing yield of conventional heads decreases, often dramatically, as the number of channels accommodated within the head increases. Employing a wider tape would require the use of an increased number of heads operating in parallel to adequately cover the increased tape width.
Another problem associated with increasing the tape width is that the head-to-tape contact pressure is very difficult to maintain uniformly from the lower tape edge to the upper tape edge. This is due to head tilt (zenith) tolerances and tape transverse cupping and tension.
The above problems are addressed in two co-pending applications which are assigned to the same assignee, Tandberg Data ASA, as the present application. A multiple tape cartridge and drive system is described in co-pending application U.S. Ser. No. 09/006,459, filed Jan. 13, 1998 ("Multiple Tape Cartridge and Drive System Wherein Tapes are Extracted from the Cartridge," Rudi), in which a mechanism is provided to withdraw the multiple tapes from the cartridge to an exterior hub. The multiple tapes are temporarily affixed to the hub and the hub is rotated to unwind the multiple tapes from the hubs in the cartridge housing. Space is provided for a number of data transfer (read/write) heads to be moved adjacent to the respective tapes in a region between the cartridge and the exterior hub, so that data transfer can take place. When a data transfer operation is completed, the tapes are detached from the exterior hub and are returned to the inside of the cartridge.
In another co-pending application, U.S. Ser. No. 09/041,689 filed Mar. 13, 1998 (Multiple Tape Cartridge and Drive for Concurrent Recording with the Tapes Disposed in the Cartridge," Rudi) the cartridge housing is provided with an access opening into which multiple data transfer heads can be simultaneously inserted. When inside the housing, each data transfer head is adjacent the tape transport path traversed by one tape, among multiple tapes, which are wound in overlapping fashion on two reels in the housing. The tapes thus always remain in the housing, even during data transfer.
Many types of robotics systems are available for cartridge handling and manipulation, such as so-called loader and library systems. For these types of systems, currently two cartridge form factors are particularly important, these being the DLT cartridge, commercially available from Quantum, and the IBM 3840/3490 cartridge. Both of these cartridges employ tape which is one-half inch in width. A large number of loader/library systems currently in use are designed to handle cartridges having a form factor corresponding to these standardized cartridges.
An advantage of these cartridge types is that they have a large available recordable surface area which is available, compared to other cartridge types and other media types. A disadvantage of these types of cartridge, however, is that the data transfer rate is limited because the number of data transfer (read/write) heads is limited which can be used in parallel for a one-half inch tape. Another disadvantage of these standardized cartridge types is that in order to obtain access to data at the end of the tape, it is necessary to unwind (or rewind) the entire tape length, which is approximately 2,000 feet. An unavoidable delay is therefore associated with obtaining access to data at this location on the tape.
Moreover, these standardized cartridges are of a type wherein the tape is extracted from the cartridge, and is wound onto a take-up hub located remote from the cartridge in the drive, and thus before the cartridge can be unloaded (removed from the drive), the tape must be completely rewound and returned to the cartridge. Moreover, the tape threading mechanism associated with cartridges of this type is very complex, and this complex threading mechanism imposes a lower limit for tape thickness, and thus also imposes a lower limit for tape length.